This application claims priority on Finnish Patent Application No. 20010129, Filed Jan. 22, 2001, the disclosure of which is incorporated by reference herein.
Not applicable.
The invention relates to a twin-wire former in a paper machine, which former includes two forming wire loops defining between themselves a twin-wire zone, and at least one dewatering box located inside one of the wire loops to remove water through said wire from the web being formed, and at least one loading blade located opposite to the dewatering box inside the other wire loop and in contact with said wire, said dewatering box including at least three successive dewatering zones.
In a gap former of a paper machine, a pulp suspension is fed into a forming gap between two forming wires, which wires are guided to curve over a forming roll and/or a forming shoe, which is equipped with a curved blade deck. In the narrowing gap, water is removed from the pulp suspension, among other things, by the action of a pressure caused by the tension of the wires, on the one hand, through the outer wire boosted by the centrifugal force and, on the other hand, through the inner wire boosted by the suction acting in the forming roll or shoe.
In the twin-wire zone after the forming gap, various kinds of web forming and dewatering elements are used, the purpose of which is to bring about pressure pulsation in the fiber layer being formed in order to promote dewatering of the web being formed and to improve its formation at the same time. U.S. Pat. No. 5,798,024 describes a state-of-the-art gap former in which a forming shoe and a loading blade unit function as dewatering and web forming elements after a forming roll. The blade deck of the forming shoe may be straight or curved. The loading blade unit includes a dewatering box provided with dewatering blades and a set of blade elements the loading of which can be controlled and which are placed inside opposite wire loops and in alternating positions with respect to one another so that the pressure pulses applied to the web by the dewatering blades and the loading blades alternate in the running direction of the web. The dewatering achieved by the loading blade unit is boosted by arranging a vacuum at at least one blade set and, preferably, at both blade sets.
The formation of the paper produced is improved by using a loading blade unit but, at the same time, it increases the porosity of paper, sometimes even to a harmful degree. Especially when making fine paper, high porosity may be a property which is attempted to be avoided. For this reason, loading blade units are not generally used when making paper grades containing highly beaten pulp and an abundance of filler. It has been found that in such cases it is preferable to use, for example, a forming shoe as the dewatering element, because it brings about lower pressure pulses in the fiber layer and affects the porosity of paper considerably less than a loading blade unit.
An aim of the invention is to reduce the above-noted problems associated with the prior art. A particular aim is to provide a novel twin-wire former by means of which efficient dewatering is achieved even at high running speeds and which makes it possible to produce paper having uniform formation and suitable porosity.
The twin-wire former according to the invention includes one or more dewatering boxes by means of which vacuum zones and vacuum-free zones are arranged in the web forming and dewatering zone, which zones alternate in the running direction of the web. One or more loading blades are arranged inside the wire loop situated opposite to the dewatering box, the loading blades being disposed such that opposite to each loading blade on the opposite side of the web there is a vacuum-free zone, which is preceded and followed by a vacuum zone. Since the vacuum zones and the loading blades are arranged to alternate in the running direction of the web in the proposed manner, the vacuum boosting dewatering and the loading blade producing a pressure pulse will never simultaneously affect the web that is being formed.
Alternation of the vacuum zones and vacuum-free zones is provided, for example, by dividing one continuous dewatering box with partitions into compartments which form several successive dewatering zones. Of these zones, every second zone is connected to a vacuum source in order to make dewatering more effective, and from every second zone water is removed mechanically without any significant vacuum.
Different vacuums can be advantageously arranged in the different vacuum zones so that the vacuum may be increased in the running direction of the web as the solids content of the web increases and dewatering becomes more difficult.
The number of loading blades and vacuum zones may vary in different applications. What is essential is that opposite to the loading blade there is always a vacuum-free zone, which is preceded and followed by a vacuum zone. In addition to this, the twin-wire former may include a pre-loading blade which precedes the dewatering box proper and which is also most advantageously placed at a location where it is opposed by a zone working without vacuum.
The invention combines characteristic features of known dewatering elements in a totally new way, so that by means of the new twin-wire former it is possible to simultaneously achieve good formation produced by the loading blades and moderate paper porosity produced by the suction forming shoe.
In the following, the invention will be described in greater detail with reference to the figures shown in the appended drawings, but the invention is not meant to be exclusively limited to the details of the figures.